This invention relates to a detection system for determining information concerning the location of objects, and which can be extended to determine movement and orientation and even physical parameters such as shape of objects in a specified environment. The invention can be applied to people and animals as well as inanimate objects such a furniture, machines, vehicles, equipment and the like, and in this connection object is intended to include any movable entity.
Location systems are known which allow the presence or absence of an object in a specified environment (such as a room) to be confirmed or denied, and relative to one or more reference points to identify where in the environment the object is located.
EP 0485879 describes a system for locating vehicles in automatic guidance transport systems. Ultrasound is employed as a distance measuring medium whilst an infra-red link allows communication between vehicles.
WO95/14241 describes a tracking system which enables a spotlight to follow a person on a stage carrying a transponder. Again infra-red signals are used to instigate ultrasonic transmissions to determine the position of the transponder and therefore the person. The spotlight is moved accordingly.
EP 0591899 describes another spotlight controlling system for tracking a moving target (actor on a stage) carrying a transponder. Here radio transmissions establish the communication link and ultrasound transmissions are employed to determine distance and position.
In our UK Patent Applications Nos. 9725760.4, 9725718.2 and 9725759.6, there is described an indoor positioning system wherein a single radio transmitter (part of a zone manager) transmits 25 radio signals each second, at the start of a 40 ms interval called a timeslot.
The radio signals, which are picked up by mobile transmitter/receivers (transponders) in rooms to which those signals propagate, contain an address, chosen for each message. by a scheduling PC.
Each mobile transponder has a unique address, and compares this with the address contained in each incoming radio message. If the addresses match, the mobile transponder emits a short pulse of ultrasound, which is detected by a set of receivers (each room has its own receiver set). These receivers include a counter, synchronised with the radio messages by means of a wired network, and it is therefore possible to determine the time-of-flight of the ultrasonic pulse from the mobile transponder to each receiver that detects it.
An estimate of the speed of sound in each room (derived from the ambient temperature) is used to calculate the corresponding transponder-receiver distance.
A non-linear model is then fitted to these distances and the known receiver positions by a regression algorithm which runs on a positioning PC (connected to the receiver set by means of a matrix manager).
A global time signal is sent by a clock generator to the zone manager and matrix manager(s). This signal ensures that the Scheduling PC knows which mobile transponder was triggered in a particular timeslot. It also ensures that, if sufficient ultrasonic signals from that mobile transponder are detected, the positioning PC dealing with the room in which the ultrasonic signals were generated knows where a mobile transponder was at the start of the timeslot.
By correlating these two items of information in a software entity called an area manager, it is possible to determine the 3D position of a mobile transponder at a particular time.
The aforesaid system subject of the above-numbered Patent Application is later described in more detail.
The system implementation described in the aforesaid Patent Applications has 40 ms timeslots, but these could be shortened to around 20 ms without affecting the basic operation of the system. However, reverberations of an ultrasonic pulse in a typical office may last up to 20 ms, therefore the timeslots cannot be made shorter than this interval. Thus considering the case of a system in which all mobile transponders are in a single room, then with say, a 15 ms timeslot, the period in which reverberations from a mobile transponder triggered in one timeslot might reach a receiver which would overlap the period in which signals from a mobile transponder triggered in the next timeslot might reach the receivers. It would then not be possible to determine which mobile transponder had generated a particular signal, and errors may be introduced into the calculation of a mobile transponder""s location by using a signal generated by another mobile transponder.
When mobile transponders are distributed between a set of rooms, however, the aforesaid limitation appears restrictive. The periods in which signals from two or more mobile transponders are in flight may overlap safely if it is certain that the signals do not overlap physically. For example, if two mobile transponders are in separate rooms, which are ultrasonically isolated from each other, then the intervals over which their signals are in flight may be overlapped without the possibility of signal cross-contamination. Similarly, in large open-plan offices, the regions in which signals are detected by receivers can be examined, and if those regions are distinct and separated by a significant distance, it can be assumed that the ultrasonic signals will not cross in space.
This consideration appears to suggest that a zone manager could address two or more mobile transponders simultaneously (or as nearly simultaneously as the radio trigger link would allow), as long as those mobile transponders were in separate rooms (or separate ultrasonic spaces).
However, bearing in mind the mobility of the transponders, as the ultrasonic signals contain no addressing information, so a set of signals detected by receivers in an ultrasonic space could have been generated by any of the addressed transponders. It would then be impossible to determine which mobile transponder was at which location.
Past location information for the mobile transponders might provide some guidance in this matter, but would not rule out the possibility that a number of the mobile transponders had been transposed.
It is therefore necessary to provide some other means by which the area manager can identify which set of detected signals was generated by which mobile transponder.
According to one aspect of the invention, there is provided a system which enables the position of each of a plurality of labelled objects in a specified environment to be determined by determining the transit time of slowly propagating energy transmitted from a transmitter on each labelled object to a plurality of receivers positioned at fixed points in or around the specified environment, and computing therefrom the actual distance of the transmitter from the receivers, wherein the transmission of the slowly propagating energy is initiated by a burst of high speed propagating energy from a master transmitter located so as to cause transmitted bursts of such high speed energy to enter the said environment, the transmitter on the object being controlled by a receiver adapted to respond to an appropriately encoded burst of such high speed energy, each said burst being encoded so that only one of the object mounted receivers is triggered by each burst (each transmitter/receiver combination being referred to as a transponder), to thereby initiate a burst of slowly propagating energy therefrom, wherein the specified environment has at least two zones substantially isolated from one another in respect of the slowly propagating energy, the times at which the transponders are triggered on objects hitherto understood to be in one zone (one set of transponders) are staggered within common timeslots with respect to the times at which the transponders are triggered on objects hitherto understood to be in another zone (another set of transponders), each timeslot being of a sufficient length to enable the receivers to respond to the slowly propagating energy transmitted by the triggered transponders in both the first zone and the or each other zone, and an algorithm is applied to verify the consistency of the transit times of the slowly propagating energy when ascribed to the different sets of transponders.
The slowly propagating energy is typically ultrasound and the high speed energy is radio waves, and in order to be able to distinguish between sets of signals from different mobile transponders, it is first assumed that two different transponders are in different ultrasound zones, the triggering times of those two different mobile transponders (one from each of the two sets) are staggered, and the system is adapted so that receivers are triggered when the first of the said two different mobile transponders is triggered, and the normal 20 ms timeslot is extended by
(mxe2x88x921)xc3x97i,
(where m is the number of mobile transponders triggered in the staggered sequence, and i is the stagger time).
A nonlinear regression calculation used to determine the 3D positions of the mobile transponders should only converge to a solution if it is given a set of consistent transponder-receiver distances and receiver positions. Thus if receiver detected signals are ascribed to the wrong mobile transponder (because it has moved from one zone into another), the pulse times-of-flight calculated for the receiver detected signals will be incorrect, because an inappropriate trigger time will have been used for the signals. The corresponding transponder-receiver distances will then be incorrect, and the nonlinear regression computation will not converge to a solution.
Therefore, given a set of n possible trigger times for mobile transponders, t1, . . . tn, in a method embodying the invention a first check is made to see that the number of sets of signals that were detected in independent ultrasound zones is the same as the number of mobile transponders that were triggered. If this is not so, no further use can be made of these readings, because two or more mobile transponders apparently occupy the same ultrasound zones, in which case signal cross-contamination could have occurred.
Then, for each set of signals s1, . . . , sn, a set of possible corresponding transmitter-receiver distances is calculated for each trigger time, d11,d12, . . . , dnn. By running a nonlinear regression calculation on each set of transmitter-receiver distances (using an algorithm which uses the positions of receivers that detect the corresponding signals), a set of results r11, r12, . . . , rnn is obtained. Each value of rxy, (where 1xe2x89xa6xc3x97xe2x89xa6n) and 1xe2x89xa6yxe2x89xa6n is either a 3D position (indicating that the set of signals sx could have been generated by a mobile transponder at time ty) or an error value (indicating that the set of signals sx could not have been generated by a mobile transponder triggered at time ty).
Ideally, one and only one set of signals will be consistent with each trigger time, making it possible to identify which mobile transponder gave rise to those signals (and hence enabling a 3D position to be ascribed to each mobile transponder).
If this is not the case, other heuristics are applied to the results to determine 3D positions for the mobile transmitters. One such heuristic might be that if one set of signals is consistent with two trigger times ta and tb, but another set of signals is consistent with trigger time ta and no other set of signals is consistent with trigger time tb, then the first set of signals was probably generated by a mobile transponder triggered at time tb.
Preferably simulations are performed to validate the heuristics used.